Field of the Invention
The present invention relates to the field of electrical communication networks and more specifically to the area of such networks ideally suited for the control of electrical functions in the vehicular environment.
Description of the Prior Art
Several protocols have been proposed for use in vehicle data communications over the past several years in order to efficiently replace the massive wiring harness that is presently used to provide electrical interconnection of the various switches, sensors, lights, motors, and electronic control modules located throughout the vehicle.
One such protocol proposed in the prior art is based on Time Division Multiplexing (TDM) in which a master timing node is used as a controller to provide a synchronized set of dedicated time slots for which each slave node is obliged to synchronize with and obtain its communication data therefrom. The dedicated time slots in the TDM each provide control information of a prescribed type and those slave nodes which are programmed to perform the prescribed function must be synchronized in such a way so as to extract information from the time slot correspondingly dedicated to that function. Each "receiver" slave node must know exactly when its control function occurs in the data stream. Accordingly, such a system is vulnerable to a failure in the master node since, in such case, the entire network will be unable to communicate.
In addition to the TDM's dependence on the master node operation, TDM systems generally have poor flexibility due to the limited allocation of time slots in the data stream. Accordingly, additions or deletions of types of receivers in a particular network require a reallocation of the time slots through programming of the master control node.
TDM systems also operate on an open loop system in which there is generally no positive acknowledgment that a particular control function has been received. Consequently, TDM system usually provide a continuous update of control data as long as a particular function is being instructed.
Some master/slave network architectures use a polling technique. This is considered an improvement over traditional TDM systems since the master can dynamically reassign time slots. However, all network transactions are dependent on a master node and redundant master nodes are required if the system is to be reliable.
A significant improvement over Time Division Multiplex networks is provided by a Bus Contention (BC) network architecture. A Bus Contention network is characterized by a unified system of nodes, each capable of accessing the network based on its own requirements. There are no "master" nodes, nor any dependence on any particular nodes for network operation.
Instead of the dedicated time slots used in TDM, BC nodes have an "address" by which they are identified. This address uniquely identifies a node to all other parts of the network system. Control messages are directed by the address to the specific node that is responsible for a particular control function.
The BC system has far greater efficiency than TDM systems because network activity is directly related to requests for activation. The bus is not burdened with the constant repetition of control signals which are not active. Messages flow between the nodes, as needed without the intervention of a master controller. Such a method allows for greater adaptability and expandability of the network. New nodes are simply connected to the bus. No reallocation of time slots is required and existing nodes are not affected.
A requirement for the BC type of network architecture is for media access resolution. Because all nodes are capable of network access (i.e., "transmitting") based on their individual needs, a method of resolving conflicts arising from simultaneous transmitter activations is required. Several different solutions are in widespread use for Local Area Network (LAN) applications. CSMA/CD (Carrier Sense Multiple Access with Collision Detection) and Token Passing schemes are most common. These, however, 2re optimized for physically (and electrically) large networks where message propagation delay between nodes is large. Collisions often are not detected until both transmitters are well into their individual message sequences. An "abort and retry" scheme is used to resolve the conflict, whereby, both messages are aborted and both transmitters must make attempts to retransmit at a later time.
A different and more efficient method of message collision resolution has been found to be suitable for small networks. It is called the "bit-wise contention resolution" technique. An example is the Philips/Signetics D.sup.2 B network protocol as disclosed in "The D.sup.2 B a One Logical Wire Bus for Consumer Applications", by C. H. Kaplinsky, et al., IEEE Transactions on Comsumer Electronics, Vol. CE-27, February 1981, pg. 102-116; "Serial Bus Structures for Automotive Applications", by A. J. Bozzini, et al., SAE Technical Paper Series No. 830536, February 28, 1983; and "A Small Area Network for Cars", by R. L. Mitchell, SAE Technical Paper Series No. 840317, February 1984.
The D.sup.2 B method relies on small network propagation delays, being much less than a single bit period. For a given size network, this places an upper bound on the potential frequency response or bit rate of the network. Because the network "looks" small electrically, each bit of a message exists at all points on the network simultaneously. Each transmitter, therefore, is aware of network activity on a bit-by-bit basis in real time. This allows a technique of message arbitration which resolves conflicts "on the fly". Messages are not destroyed in the arbitration process; rather, the network "sees" one of the conflicting messages as being valid. The losing transmitter detects this and tries again as soon as the first message has passed.
The general BC network architecture described above, where each node has a unique address, is characterized by messages which have specific transmit origins and receive destinations. These "node-to-node" messages contain specific references to the physical address of both the transmitter and receiver, along with a data field which contains an encoded description of a particular activity to take place. While BC network configurations are a significant improvement over TDM systems, there are still aspects of network operations unique to automotive requirements which are not completely addressed by the node-to-node schemes.